Amorphous carbon films having diamond-like properties of extreme hardness, extremely low electrical conductivity, low coefficients of friction, and optical transparency over a wide range of wavelengths, have widespread applications as optical coatings, low friction, anti-corrosion coatings and wear-resistant coatings and in various other applications such as surface finishing and in semiconductor manufacturing.
Diamond-like carbon is a non-crystalline, or amorphous, material having two types of carbon-carbon bonds, i.e., hexagonal graphite bonds (sp.sup.2) and tetrahedral diamond bonds (sp.sup.3). Thus, diamond-like carbon has both limited long range order and two types of short range order.
Diamond-like carbon may be hydrogenated or non-hydrogenated. Hydrogenated diamond-like carbon is produced from a hydrocarbon gas mixture using various energy sources, e.g., DC discharge, microwave and RF energy, oxyacetylene torches, and hot filaments. Non-hydrogenated diamond-like coatings can be produced using magnetron sputtering, electron beam evaporation, laser ablation and mass filtered carbon-ion beam deposition techniques, each of which has a very low deposition rate. Each method for producing the non-hydrogenated diamond-like carbon produces a hard carbon coating but the coatings have differing sp.sup.3 /sp.sup.2 bond ratios and, thus, the structural and physical characteristics differ.
The sp.sup.3 /sp.sup.2 bond ratio can be estimated from the plasmon energy determined by electron energy loss spectroscopy. The plasmon energy is proportional to the atom density and diamond has a greater atom density and plasmon energy than graphite. Polycrystalline graphite has a plasmon energy loss of about 25 electron volts (eV). Diamond has a plasmon energy loss of about 33 eV. Non-hydrogenated diamond-like carbon has a plasmon energy loss between about 26 and 32 eV, the higher plasmon energies corresponding to higher atom densities which are believed to be due to an increased sp.sup.3 bonding component.
Hydrogen-free non-crystalline diamond-like carbon coatings can also be produced using cathodic arc plasma deposition, a process which provides high deposition rates and allows control over the incident ion kinetic energy and the substrate temperatures. The sp.sup.3 /sp.sup.2 bond ratio is believed to be dependent on the incident ion kinetic energy. Cathodic arc plasma deposition can produce diamond-like carbon coatings with higher plasmon energies than alternative processes, thus, yielding higher sp.sup.3 /sp.sup.2 bond ratios.
A cathodic arc discharge occurs when a high current power source is connected between two sufficiently conductive electrodes and the electrodes are momentarily in contact, either physically or by another discharge. Arc spots form on the cathode surface as the electrodes are separated. These small, luminous regions are often very mobile and move rapidly over the cathode surface. Due to the high current density contained in each spot, rapid ebullition of the cathode material occurs, and this plasma material can be confined, transported using magnetic fields and deposited on substrates. The current density at each spot can reach 100,000 amperes per square centimeter and this contributes to the ionization of much of the outflowing vapor.
One of the major problems associated with cathodic arc discharges is the production of macroparticles. Macroparticles are droplets or solid particles of the consumable cathode which range in size from about 0.1 micron to greater than about 50 microns, most being between about 0.5 and 20 microns. These macro particles are deposited with the plasma to produce unwanted particles in the final coating. Macroparticle production is particularly undesirable for diamond-like carbon coatings because they are graphite and become embedded in the diamond-like carbon coating. Much attention has been given to the removal of these particles.
U.S. Pat. No. 4,452,686 (Axenov et at.) describes one means of macroparticle removal via filtration. A cylindrically shaped current coil and a central axially mounted, football-shaped coil produce a magnetic field that guides the plasma around the central coil and through the plasma guide system. Macroparticles, which are much heavier than the plasma particles are not guided by the magnetic field and, thus, are blocked by the central coil. Downstream, the plasma macroparticle density is significantly reduced.
I. I. Aksenov et at., "Transport of Plasma Streams in a Curvilinear Plasma Optics System", Soviet Journal Physics, 4(4), July-August 1978, pp. 425-428, describes a vacuum apparatus using curvilinear magnetic and electric fields to steer and focus the plasma and remove macroparticles from the plasma stream.
Although such filtration means do reduce the macroparticle density in the coatings, they do not completely remove all macroparticles. Typically, the larger macroparticles are filtered, but the submicron particles are not completely filtered. This can occur as the submicron particles bounce around in the vacuum system. This can also result from momentum transfer from the plasma to the particles. Furthermore, plasma filters typically reduce the plasma flux by 30 to 50 percent, leading to reduced deposition rates.